1. Field of the Invention
The present invention generally relates to mooring systems for offshore floaters. More particularly, it relates to mooring systems having subsea spring buoys and adapted for use with submersible floaters.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
The two main mooring system concepts usable for deep water are semi-taut and taut systems. A taut system always has a positive uplift force on the anchor in any (non-damaged) condition and has limited geometric stiffness, i.e., the stiffness of the system is mostly determined by the stiffness of the line material. Typically, the line material has a relative high strength-to-weight ratio.
The taut mooring systems of the prior art have not been used in a disconnectable mooring system, such as a Buoyant Turret Mooring system. Moreover, the taut mooring systems of the prior are have not been used in combination with spring buoys.
A Buoyant Turret Mooring (BTM) is a disconnectable turret mooring system comprising a disconnectable mooring buoy and a fixed turret structure located in the forepeak of a floating vessel such as a tanker. The mooring buoy is fixed to the seabed by catenary anchor legs, supports the crude oil and gas risers, and is connected by means of a structural connector to the fixed turret. The fixed turret extends up through the tanker, supported on a weathervaning bearing and contains the reconnection winch, flow lines, control manifolds and fluid swivels located above the main deck. (APL systems do not necessarily have swivels above the main deck.)
These systems have been developed for areas where typhoon, hurricane or icebergs are a danger for an FPSO or FSO and, primarily for safety reasons, rapid disconnection/reconnection is required.
Disconnection and reconnection operations may be carried out from the tanker without external intervention. When disconnected, the mooring buoy sinks to neutral buoyancy under water and the FPSO can sail away.
A Floating Production Storage and Offloading system (FPSO) is a floating facility typically installed on the surface above or close to an offshore oil and/or gas field to receive, process, store and export hydrocarbons. It comprises a floater, which is often a converted tanker, permanently moored on site. The cargo capacity of the vessel is used as buffer storage for the oil produced. The process facilities (topsides) and accommodations are installed on the floater. The mooring configuration may be of the spread mooring type or a single point mooring system, generally a turret.
The high pressure mixture of produced fluids is delivered to the process facilities mounted on the deck of the tanker, where the oil, gas and water are separated. The water is discharged overboard or reinjected into the reservoir after treatment to eliminate contaminants. The stabilized crude oil is stored in the cargo tanks and subsequently transferred into shuttle tankers either via a CALM buoy or by lying side-by-side or in tandem to the FPSO. The gas may be used for enhancing the liquid production through gas lift, and/or for energy production onboard the vessel. The remainder of the gas may be compressed and transported by pipeline to shore or reinjected into the reservoir.
A Catenary Anchor Leg Mooring (CALM) is a floating buoy that performs the dual function of keeping a shuttle tanker moored on a single point and transferring fluids (generally crude oil or refined products) while allowing the ship to weathervane. It consists of a circular floating buoy anchored by means of multiple chain/polyester legs fixed to the seabed by either conventional anchor legs or piles. The buoy itself is free to move up and down, sideways and in pitching and rolling motions. The shuttle tanker is moored via hawsers to the turntable on the buoy. The tanker may be loaded or offloaded by means of flexible marine hoses from the buoy to the vessel's manifold. The connection between the piping inside the buoy and the subsea pipeline may be by means of flexible hoses.
In a Turret Mooring System, the turret system is integrated into (internal turret) or attached to the hull of the tanker, in most cases near the bow, (external turret) and allows the tanker to weathervane around it and thereby take up the line of least resistance to the combined forces of wind, waves and current. A high pressure oil and gas swivel stack is mounted onto the mooring system. This swivel stack is the connection between the risers from the subsea flowlines on the seabed to the piping onboard the vessel. It allows the flow of oil, gas and water onto the unit to continue without interruption while the FPSO weathervanes. For reasons of size and cost, the number of swivels is kept to a minimum, and therefore the flow of oil and gas has to be manifolded in the turret area, particularly when the system produces from a large number of separate wells.
The turret mooring and high pressure swivel stack are thus essential components of an FPSO.
A Single Point Mooring (SPM) is a mooring system which enables the vessel to weathervane whilst it loads or unloads hydrocarbons, chemicals or fresh water. The two categories of SPMs are:                a single point mooring buoy or tower that is designed for use by any trading shuttle tanker, and is thus independent of the vessel;        a system, such as a turret mooring, that is incorporated within a vessel such as an FSO or FPSO.        
A semi-taut mooring system is a combination of two segments that have different properties. The first segment is connected to the anchor. This segment has a lower strength-to-weight ratio and most commonly is a chain. It provides geometric stiffness though catenary behavior and lay-down on the sea bed. It decreases vertical loads on the anchor and prevents the second segment from touching the sea bed.
In a semi-taut mooring system, one end of the second segment is connected to the first segment and the other end is connected to the floater. This segment has a higher strength-to-weight ratio, like polyester fiber rope, and therefore exhibits limited catenary behavior. The main contribution to stiffness from this segment is the stiffness of the material. For a semi-taut mooring system, the total line stiffness therefore is determined by both the material stiffness and geometric stiffness.
The disconnectable mooring systems of the prior art comprise a semi-taut mooring system, in some cases aided by spring buoys in the top segment. For these systems, geometric stiffness is determined by two effects: the catenary behavior of the bottom segment and the influence on line geometry by the spring buoy. These two effects are antagonists, decreasing one another's effectiveness.